1. Field of the Invention
The present invention relates to a method of forming a powder image on a recording medium, and an apparatus therefor. The method and apparatus use a toner carrier for carrying powdery toner in a layer or layers thereon, a counter electrode, and a flight control member having a plurality of independent or continuous extremely small holes, or microholes, and a plurality of control electrodes for controlling the passage of toner through the microholes. Voltages are applied to the control electrodes in accordance with an image signal in order to cause the toner to fly from the toner carrier toward the counter electrode via the microholes. The toner deposits on a recording medium intervening between the flight control member and the counter electrode, forming an image on the recording medium.
2. Discussion of the Background
An image recording method using powder and called direct toning or toner projection is conventional. In this kind of image forming method, a voltage is applied to image electrodes surrounding holes or slits so as to cause charged toner to move (fly) via the holes or the slits. As a result, the toner directly forms an image on a paper or similar recording medium.
The above image recording method has been proposed in various forms in the past. For example, Japanese Patent Publication No. 44-26333 discloses an apparatus including a counter electrode, a recording medium, a control grid, a mesh electrode and a fur brush sequentially arranged in this order. The fur brush feeds insulative toner to the mesh electrode. The insulative toner is charged by friction due to the rotation of the brush, and then accelerated toward the counter electrode by an electric field formed between the mesh electrode and the counter electrode. As a result, the toner forms an image on the recording medium, flying via the control grid. When the value of an electric signal applied to the control grid is varied, an electric field between the mesh electrode and the control grid is inverted. This prevents the toner from flying and forms a background (white) on the recording medium. Further, the value of the electric signal may be increased or decreased in order to vary image density.
However, the above apparatus using the insulative toner needs a large scale device for rotating the brush which charges the toner by friction. While the insulative toner may be charged in the same manner as in a conventional developing system using a single- or two-ingredient type developer, a large scale device including a carrier, agitating mechanism and developing roller is also needed. In addition, because frictional charging generates heat, use must be made of resin having a relatively high melting point as mother resin of the toner. This makes it difficult to lower a fixing temperature and therefore to save energy.
Japanese Patent Laid-Open Publication No. 58-44456 proposes an apparatus using conductive magnetic toner in place of the insulative toner needing frictional charging. The apparatus includes a control electrode, a base electrode and a toner carrier or toner conveying means sequentially arranged in this order. The control electrode and base electrode sandwich a counter electrode, a recording medium, and an insulating member. The control member and base electrode are formed with holes for allowing the toner to pass therethrough. A DC electric field is formed between the base electrode and the toner carrier so as to cause the toner on the toner carrier to form a cloud. While a DC electric field is formed between the base electrode and the counter electrode, a voltage for image formation is applied to signal electrodes. As a result, the toner in the form of a cloud forms an image on the recording medium, flying via the holes. The conductive toner does not have to be charged beforehand. Such toner can be easily charged by charge injection based on an outside electric field.
The toner carrier of the apparatus using the conductive magnetic toner carries the toner thereon, relying on a magnetic force. However, because a transparent magnetic material has not been accomplished yet, the apparatus cannot produce a full-color print by laminating transparent color toners.
Japanese Patent Publication No. 6-47298 teaches an apparatus having a counter electrode, a recording medium, a flight control member, and a toner carrier sequentially arranged in this order. The flight control member h a s microholes and control electrodes surrounding them. The toner carrier carries conductive toner in a layer thereon. While a DC electric field is formed between the toner carrier and the counter electrode, the voltage state of the individual control electrode is switched in accordance with an image signal to one for causing the toner to fly from the toner carrier toward the recording medium or one for preventing it from flying. When the control electrode is switched to the former voltage state, charge is induced on the conductive toner and causes it to fly so as to form an image on the recording medium. This kind of scheme, however, has some problems left unsolved, as follows.
(1) In the voltage state preventing the toner from flying, the same potential is applied to the toner carrier and control electrodes. Therefore, if the voltage state is switched from one causing the toner to fly to one preventing it from flying before the toner left the toner carrier in the former voltage state reaches the recording medium, the toner will simply float because the toner conveying force is lost. The floating toner smears the inside of the apparatus and deposits on the control electrodes to result in the need for cleaning. The document clearly describes that the conductive toner has already deposited on the recording medium when the latter voltage state is set up. It follows that during image formation the former voltage must be continuously applied to the control electrodes until the toner left the toner carrier reaches the recording medium. The duration of this voltage effects the recording speed available with the apparatus.
(2) When the resistance of, e.g., a paper decreases due to a hot and humid environment, it is likely that charge of opposite polarity is injected from the paper into the toner deposited thereon, charging the toner to the opposite polarity. When an electrostatic force ascribable to the charge of opposite polarity and acting toward the toner carrier exceeds attraction acting between the recording medium and the toner, i.e., van der Waals's forces, the toner flies in the reverse direction and lowers image density to a noticeable degree.
(3) The toner flying in the reverse direction elastically collides against the toner flying toward the recording member. As a result, the toner flies in the horizontal direction to a substantial distance. This part of the toner is scattered around a desired image on the recording medium and blurs the boundary between the image and the background. The resulting image lacks sharpness.
(4) If the toner left the toner carrier deposits on the control electrodes, charge of opposite polarity is injected into the toner and causes the toner to fly inversely toward the toner carrier. As a result, the toner flies back and forth repeatedly. The toner is therefore apt to stop the microholes of the flight control member and to deposit on the counter electrode and control electrodes via the microholes, resulting in defective images. Consequently, a cleaning step is necessary which lowers the printing speed. In addition, a shield must be provided in order to prevent the toner flying back and forth from smearing the inside of the apparatus, increasing the size and cost of the apparatus.
(5) When the conductive toner is deposited on the recording medium in layers, electrostatic induction occurs due to the electric field around the toner. Therefore, the charge of the toner forming the second and successive layers on the recording medium migrates toward the first layer close to the counter electrode. As a result, the toner of the second and successive layers are charged to the opposite polarity and caused to fly reversely. It follows that basically only a single layer is available with the conductive toner. Consequently, a full-color image in the form of a laminate of yellow, magenta and cyan toners cannot be formed.
(6) When charge is repeatedly injected into the toner, electrons are deeply trapped in the portions of the toner where the energy level is low. In this condition, the toner is charged to either polarity and prevented from flying due to the increase in electrostatic adhesion. Also, the toner hitting against the toner carrier and control electrodes repeatedly is pulverized and loses conductivity, eventually failing to fly.
(7) The control electrodes are arranged in an array in the direction perpendicular to the direction in which the recording medium is conveyed. The control electrodes or the control grids have their voltage states switched at the same time in accordance with an image signal while the recording medium is in transport. When the control grids or control electrodes with such a structure and the above voltage application scheme are used, the required number of IC (Integrated Circuit) drivers increases in proportion to the number of control grids or that of control electrodes. For example, when a sheet of size A4 having a width of 210 mm is fed in a vertically long position, and when the resolution is 600 dpi (dots per inch), 4,960 image electrodes are driven. Assume that an IC driver having thirty-two output terminals and capable of driving thirty-two image electrodes is used. Then, the number of IC drivers necessary to drive the 4,960 image electrodes is as great as 155 in total. In the case of a color printer, as many as 620 IC drivers are required.
Assume an image forming apparatus using the direct toning or toner projection scheme forms a color image. Then, use is made of, e.g., magenta, cyan, yellow and black toners. In a black-and-white mode, only the black toner is used. Therefore, when the apparatus selectively operable in a color mode and a black-and-white mode is used to form a black-and-white mode, only one of the flight control members assigned to the black toner is used while the other flight control members simply remain idle. In offices, black-and-white prints are usually predominant over color print, so that image forming sections other than one assigned to the black toner are idle most of the time. Generally, the cost of this kind of apparatus is mostly occupied by IC drivers for applying a voltage to the control electrodes in accordance with an image signal. It follows that in the black-and-white mode most of the IC drivers are not used, wasting the greater part of the printer cost. This is also true when an image forming apparatus operable with a different kinds of toners forms images by using only a particular kind of toner.